Trilateration and distance-measuring networks have been established on 12 potentially active volcanoes in the Pacific Coast States beginning in 1980. These networks, once the baseline information has been collected, can detect surface deformation that may reflect magma movement up the conduit. The rates of deformation increase as magma approaches the surface, and these measurements can therefore help determine where and when an eruption may occur (Lipman and others, 1981). Before 1980, electronic distance meters (EDM's) had been used primarily to monitor horizontal deformation during inflation and deflation of shield volcanoes (Kinoshita and others, 1974). Little horizontal deformation monitoring had been attempted on stratovolcanoes with the exception of Usu volcano, Japan, where up to 160 meters of movement was observed using trilateration techniques to monitor cyrptodomes forming in the summit area in 1977-78 (Yokoyama and others, 1981). Several distances at Mount St. Helens and Mount Hood were measured in 1972 but were not remeasured prior to the reawakening of Mount St. Helens in 1980. Several distances were measured twice at Mount Hood in the summer of 1980, owing to an earthquake swarm, but no complete network was established and no significant changes were observed.
In mid-April 1980, measurements of distances and angles were initiated at Mount St. Helens, primarily to monitor the rate of deformation of the bulge on the north side of the volcano. Displacements of 1.4-1.6 meters/day were measured on the bulge prior to May 18, but there was little or no significant change outside the bulge area (Lipman and others, 1981). A complete EDM network was established at Mount St. Helens shortly after the catastrophic eruption on May 18, 1980 (Swanson and others, 1981; Iwatsubo, Topinka, and Swanson, chapter 8).
Monitoring networks were established at other volcanoes in the Pacific Coast States, including Augustine Island, Alaska, between 1981 and 1989. All networks, except the Medicine Lake and partially completed Mammoth Lake networks (both installed 1989), have been reoccupied at least once. Periodic reoccupation of these networks is planned as part of an overall long-term monitoring program. Reoccupation serves as a check on previous data, solidifies the baseline information, and provides an assessment of the state of the volcano.
Techniques and equipment developed in the 1980's have helped to reduce costs for monitoring volcanoes without compromising the precision of the data. We explain in this report the techniques and procedures used by Cascades Volcano Observatory (CVO) scientists to measure distances at volcanoes. For further information on general trilateration and distance-measuring techniques, see Brinker and Minnick (1987) and Davis and others (1981).
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